The use of a PMR (Professional Mobile Radio) trunked narrowband land radiocommunications network, for example of the DMR (Digital Mobile Radio) or TETRA (Terrestrial Trunked Radio) type, for critical public security mission services is known.
Bringing together fixed and mobile terminals in groups in the PMR professional mobile radio network so that any terminal that is a member of a group can transmit voice data or sometimes short messages at higher speeds to the other terminals who are members of the group through the network and receive data from any other terminal belonging to the group is also known. Fixed and mobile terminals of a same group can only communicate with each other. Trunking in a base station of the professional mobile radio network is carried out in the form of channels where the number and speeds are determined to securely meet the need for communication and radio coverage resources of the mobile terminals.
Further, in a particular asynchronous operating mode called DMO (Direct Mode of Operation), the mobile terminals of the same group communicate directly with each other without going through a base station of the trunked radiocommunications network. Thus, in the DMO mode, no base station or repeater is used, but the range is limited to a few hundred meters. This mode makes it possible to communicate in areas outside radio coverage (for example in underground parking lots).
In such trunked radiocommunications network operating in DMO mode, each frequency channel—allocated to a group of terminals—is independent, at least from the viewpoint of synchronization, of the other channels. Thus, synchronization is only between the terminals of the same channel. The usage frequencies of each channel are determined in advance. Only the start times of frames and the sequencing of frames are synchronous and managed for each group.
When such a channel is allocated, any terminal of a group can use the resource at any time, which means that a protocol must be used to monitor and detect collisions. In the event of a collision, the message is repeated on the channel at the end of a random time. Monitoring of collisions requires arbitration.
Thus, when frequency channels exist and are specifically dedicated to the DMO operating mode in a trunked narrowband radiocommunications network operating in DMO mode, that mode requires the management, monitoring and detection of collisions, and thus has a lower useful speed.